Our studies are concerned with the mechanism of action of neurotransmitters, using the glutathione feeding response of Hydra as a model. Analogs to glutathione (activator) and glutamic acid (inhibitor) are synthesized and tested for their activator and/or inhibitor activity in a quantitative bioassay utilizing intact Hydra. The response (duration of time the mouth remains open in the presence of an activator) is dependent on the concentration of glutathione; is saturable; and is amenable to kinetic analysis. Environmental effects on the bioassay are also under study. These include the effect of sodium ions and of pH. Further studies include the development of an in vitro binding assay and an affinity chromatographic system to aid in the purification of the glutathione receptor.